Batteries and Battery Monitors Part 4 – Battery Monitor & The Shunt

Our Battery Monitor

I have installed a Victron Energy BMV-712 Smart battery monitor, but there are many other options from other manufacturers, see the listing at the end of this article.

I selected this one as it gave me the opportunity to connect to my onboard Raspberry Pi which I am using to give some smart boat features and provide live location tracking for my website in the future.

Our Battery Monitor Display
Our Battery Monitor Display

The battery monitor comes in two parts, a display which provides a readout of the battery status and a means of programming/setting up the system for your configuration.

The Shunt

Victron Shunt
Victron Shunt (c)Victron

The second part is called a shunt, this needs to be placed, in our case, on the negative side of the battery bank, so that everything must pass through it to reach the battery, there should be no other connections to the negative side of the batteries else your measurements will never be accurate.

A shunt is a resistor of very low but known value that is placed in parallel with a voltmeter so that all the current being measured flows through it. The voltage drop across the shunt’s resistor is measured; this voltage drop across the shunt is proportional to the current flowing and can then be calculated using Ohms law (Current = Volts / Resistance).

Shunts are rated for the maximum current they can measure, in our case 500A which at 12V is 6000W, more than enough for our boat.

Basic Battery Monitor Wiring
Basic Battery Monitor Wiring

The battery monitor also has an extra cable that connects to the positive side of the battery bank to power the electronics and the display, but also to measure the current battery voltage, we have an extra wire connected to the starter battery, so we can monitor its voltage as well.

Peukert’s Law

To then calculate the remaining capacity of the battery, the monitor uses an adaptation of Peukert’s Law** which can be used to calculate the capacity of lead acid batteries at different rates of discharge.  As we discussed earlier, the discharge rates affect the battery capacity.

** Developed by Wilhelm Peukert (1855-1932) Peukert’s law is used to calculate the batteries deliverable capacity at the current given rate of discharge, His law describes the batteries capacity at a constant discharge until it reaches its cut off voltage, below which you can damage your battery, this constant is called ‘K’, for example K=1.25 is used for our flooded lead acid batteries. There are however some limitations to this law as it does not consider the batteries temperature or age. I expect each monitor manufacturer modifies this to consider these extra factors when displaying the results, our system records each battery charge/discharge cycle.

The capacity of a battery falls at higher rates of discharge because the chemical reaction within the battery reaches its maximum speed for the given plate size and therefore the voltage drops. If left to recover, that missing capacity will return.

Using these calculations, a battery monitor can calculate the available power remaining while the battery is in use (under load) and as that load changes or even as the battery is charging, it can display the current State of Charge (SOC).

Now you know what is happening?

Armed with this information, you can then decide how you want to operate your boat and if you will need to start the engine to charge the batteries. One of our future projects is to work out the size we need for some solar panels, and we will use the data from the battery monitor to help calculate the size system we need, but that is a topic for another day

Battery Monitors Suppliers

Victron – www.victron.com
NASA Marine Instruments – www.nasamarine.com
Advanced Yacht Systems – www.advanceyacht.co.uk
Simarine – simarine.net/
Votronic – www.votronic.de

Batteries and Battery Monitors Part 1 – Why we needed one

In this multi part post we are going to look at why you need a battery monitor in the first place, the alternative types of batteries available and then how they are rated for capacity.  It is then useful to know how your battery charger works before rounding up by looking at what a battery monitor can calculate for you.

Our Battery Monitor
Our Battery Monitor

Having been electric car users for a number of years we understand how careful use of our batteries can extend our range. It still came as a surprise to us when we hired a boat and the lights would go out quite early if we stayed for a couple of days without running the engine. Hire boats are designed to be used, they have just enough battery capacity to do the job, they normally don’t include solar panels, or even a larger inverter.

Part of our upgrades and modification of our ex-hire boat, floydtilla.co.uk, was to add an additional leisure battery and upgrade the inverter to an 3500W inverter charger with a shoreline charger.

The batteries are one of the most expensive consumables on your canal boat, good management of them can increase their life by many years, and the best way of keeping them in good condition is to know how much power you have available.

Why voltage is not enough

When I started looking at battery monitors, it was suggested “Why bother all you need is a voltmeter to see your state of charge” and while mostly true, you can tell a lot from the battery voltage. The voltage will drop if you are using the battery, and most of us cannot be bothered to isolate the batteries before taking a reading.

A good battery monitor not only displays the current battery voltage, but it also displays the current in amps and the power in watts being used. This can be extremely useful when trying to find out how much power you are using and on what device. On the monitor we installed we also get a time remaining display which shows us in hour and minutes how long the battery will last at the current usage.

When battery size is not the FULL answer?

The biggest confusion with battery monitors and what causes the most misunderstanding is the State of Charge/Voltage displays. In use, a battery’s voltage will drop depending on the load it’s providing, and batteries do not really have a fixed capacity; it all depends on how you use them. Even when new, it is unlikely your 400Ah battery bank will give you a true 400Ah, and then will only degrade as they get older.

Your battery’s capacity will vary widely depending on how fast you discharge it. The slower you discharge the battery the higher its potential capacity and the faster you discharge it the lower its capacity.

We tend to leave the battery monitor on its “Time Remaining” display as this very quickly shows me if what I am doing will cause me a problem, and if I will need to run the engine before we move on.

Inverters eat your power

Inverters use quite a lot of energy converting your batteries 12V or 24V up to the 240v, most seem to be between 80-90% efficient, so that would mean that for 200w you are really consuming up to 240W. Inverters become more efficient when they reach over approximately 30% of their capacity, so best not to use them to just power your mains phone charger.

Monitoring what’s using your battery

The battery monitor we fitted has a Bluetooth connection to a smartphone (currently both Apple and Android apps are available). Using this, I captured some screenshots showing the battery usage (Click on the image to enlarge).

Fridge and Laptop usage Inverter Switched On Vacuum Cleaner
Just the Fridge and Laptop Inverter switched on Vacuuming

When using our mains vacuum cleaner via our inverter from the battery bank, the monitor dropped to just under 2 hours left at that usage, but this soon returned to days after I had stopped cleaning. Even I would find it hard to hoover for 2 hours on our narrow boat.

Different battery technologies have different profiles when it comes to their capacities. Lead Acid batteries should only be discharged to 50%, else permanent damage can occur, so even when you have a battery that states 100Ah capacity, you may only have a usable 50Ah capacity, and this will drop as the battery ages.

Your Battery C-Rating

Batteries are often marked with a C rating which gives the number of hours that the battery is designed to deliver its fully capacity over. Our Leoch Adventurer SFL-110 batteries have two C rating marked for their capacity 100Ah C100 and 85Ah C20.

Battery C Rating
Battery C Rating

This means that the battery will provide 100Ah if discharged over 100 hours or just 85Ah if discharged over 20 hours.

e.g.

C20 85Ah The battery can discharge at a rate of 4.25 amp for 20 hours 4.25 X 20 = 85Ah
C100 100Ah The battery can discharge at a rate of 1 amp for 100 hours 1 X 100 = 100Ah

So far, we have been talking about Lead Acid Batteries, which are still the most common batteries found onboard and the most limited in terms of usable capacity.  In the next part of this series, we will look at the available battery options and their costs.